You must put your ownCloud server into maintenance mode before performing
upgrades, and for performing troubleshooting and maintenance. Please
see Using the occ Command to learn how to put your server into
the various maintenance modes (maintenance:mode,maintenance:singleuser,
and maintenance:repair) with the occ command.

maintenance:mode locks the sessions of logged-in users and prevents new
logins. This is the mode to use for upgrades. You must run occ as the HTTP user,
like this example on Ubuntu Linux:

$ sudo -u www-data php occ maintenance:mode --on

You may also put your
server into this mode by editing config/config.php. Change
"maintenance"=>false to "maintenance"=>true: